The present invention relates in general to radio reception of a relatively weak desired signal in the presence of a relatively strong undesired signal at a nearby frequency, and, more specifically, to controlling a front-end attenuator to maximize a back-end signal-to-noise ratio to thereby prevent overloading of a front-end amplifier by the strong undesired signal.
In a heterodyne radio receiver, it is well known that a front-end radio frequency (RF) stage typically has a passband significantly greater than the bandwidth of the desired signal. Especially at high frequencies, it is difficult and expensive to filter out undesired signals. Therefore, filtering to select only the desired signal is usually performed in a subsequent intermediate frequency (IF) stage and/or at baseband.
Due to the low voltage levels of an antenna signal, the RF front end includes one or more stages of RF amplification. There may be an initial stage providing a fixed gain, for example. To ensure that an RF signal of appropriate magnitude is provided by the RF stage to the subsequent IF stage, an automatic gain control (AGC) loop is typically connected to a variable-gain RF amplifier for at least one stage of RF amplification. The amplifier AGC control optimizes the overall level within the RF amplifier passband (i.e., the AGC cannot optimize the RF level of the desired signal individually). When receiving a relatively weak, desired radio signal in the presence of a relatively strong, undesired signal at a nearby frequency (e.g., an adjacent channel or an alternate channel), interference with the desired signal may greatly degrade the reception quality of the desired signal. An especially strong signal can also saturate or overload one or more RF amplifier stages so that a desired signal may be completely lost.
The foregoing problem can be particularly acute in the context of a radio receiver for the satellite digital audio radio service (S-DARS) which utilizes a combination of satellite transmitters and terrestrial repeaters. The terrestrial repeaters fill in gaps where reception from the satellites is reduced (such as in an urban area where the receiver is shielded from the satellites by buildings). Two S-DARS broadcast systems are currently licensed for operation in the United States. Both the satellite and terrestrial transmissions of these services are contained within an allocated spectrum of 2320 to 2345 MHz. The satellite and terrestrial signals of each particular service are broadcast at distinct frequencies within this band.
In general, the magnitude of the received signal strength for the satellite signal is relatively modest but does not vary greatly as long as the path to the satellite is unobstructed. On the other hand, the received signal strength for the repeater signal varies greatly between very large and zero depending upon the distance of the nearest terrestrial broadcast tower.
An S-DARS receiver typically uses two separate antennas, one for reception of the satellite signals and one for reception of the terrestrial signals. Both antennas may be physically packaged together in a common mechanical housing for ease of installation in an automobile, for example. The satellite and terrestrial signals are processed in two separate signal paths in the receiver front-end circuitry. The separate signals are demodulated separately at the receiver back-end and are then combined in a known manner to achieve a best overall signal reproduction, such as in a maximum ratio combiner.
Due to the differences in elevation of the normal lines of sight to the satellites and the terrestrial repeaters, the separate antennas typically have different reception patterns. The satellite antenna has its greatest gain with respect to RF signals arriving from a high elevation, while the terrestrial antenna has its greatest gain with respect to RF signals arriving from a lower elevation. Each antenna signal is typically fed to an RF front end including a respective variable gain RF amplifier for each signal. Since the satellite signals generally have a relatively lower average signal strength, the RF amplifier in the satellite signal path may typically be provided with a relatively greater overall gain capacity than in the terrestrial path.
A disadvantage of the foregoing S-DARS system is that a significant amount of signal from the terrestrial repeaters can potentially appear at the output of the satellite antenna. For example, repeaters may be deployed at the tops of buildings so that an automobile near the building may have its satellite antenna oriented with its greatest sensitivity pointed toward the terrestrial antenna. Due to the close proximity to the repeater, the signal strength of any undesired signals from the repeater that are picked up by the satellite antenna can easily be much greater than the expected signal strength for which the satellite signal path was designed.
Terrestrial repeaters of the two S-DARS systems are not typically located near one another. Consequently, when a receiver of one system is close to a terrestrial repeater of the other system then it is primarily depending upon a satellite for reception of its desired system. In that case, the stronger, undesired signal from the repeater may impede reception of the desired signal from the satellite. If the amount of the unwanted repeater signal is large enough (as in the case where the vehicle is very near an interfering terrestrial repeater tower), it can potentially drive the front-end circuitry of the satellite receiver path into non-linear operation (i.e., saturation). When this occurs, the signal-to-noise ratio of the received satellite signal greatly decreases and satellite reception may be totally lost.
In order to minimize saturation in the satellite signal path, a certain amount of attenuation can be added to the satellite signal path. Previously, a variable attenuator at the input of the satellite signal path and driving a fixed gain RF amplifier has been controlled in a manner to limit RF amplifier output voltage. Although some improvement is obtained, there continue to be numerous occurrences wherein the satellite signals are lost or greatly distorted. Furthermore, circuitry to measure and limit the RF amplifier voltage adds expense to the receiver.